Chemistry Reference
In-Depth Information
The solvent was generally benzene and the temperature was normally 25
◦
C. The radicals were generated
in the cavity of an EPR spectrometer and decay was monitored after shuttering the light. Diaryliminoxyls
and alkylaryliminoxyls decayed with the expected second order kinetics, for example, Equation 6.3:
NO
•
2k
3
298K
10
3
M
−
1
s
−
1
=
→
(
=
.
×
)
2Ph
2
C
products
2
5
(6.3)
Surprisingly, most of the dialkyliminoxyls decayed with first order kinetics and with remarkably similar
decay rate constants (Equation 6.4), for example,
8
k
4
298K
10
−
2
s
−
1
for R,R
=
4
×
=
(CH
3
)
2
;(CD
3
)
2
;
(PhCH
2
)
2
;CH
3
,PhCH
2
;
c
-C
5
H
10
; and others.
R,R
C
=
NO
•
→
products
(6.4)
However, the concentrations of these dialkyliminoxyls under steady UV irradiation was proportional to
the
square root
of the light intensity. The first order decay kinetics arose because the iminoxyls were
in equilibrium with their dimers and (most probably)
8
the decay of the dimers was rate controlling,
Equation 6.5:
2R,R
C
=
NO
•
R,R
C
=
{
NO
}
2
→
products
(6.5)
The existence of these radical/dimer equilibria was demonstrated by showing that during the decay of
an iminoxyl radical, the iminoxyl concentration could be temporarily increased by rapidly raising the
temperature of the sample by 10
◦
C. Following this temperature-induced increase in concentration, the sub-
sequent decay rate of the iminoxyl was faster than at the initial, lower temperature.
8
Di-isopropyliminoxyl
10
−
3
s
−
1
10
−
7
-1
10
−
5
M) but
decayed with first order kinetics (
k
4
=
7
×
)
at low concentrations (2
×
×
with second order kinetics (2
k
298K
10
−
3
M).
A blue “head-to-tail” dimer of this iminoxyl was isolated (together with other products) by oxidation of
di-isopropyl ketoxime with silver(I) oxide (Ag
2
O), Equation 6.6
8
:
10
2
M
−
1
s
−
1
10
−
4
-1
=
1
.
9
×
)
at high concentrations (1
×
.
3
×
2
{
(
CH
3
)
2
CH
}
2
C
=
NOH
+
Ag
2
O
→{
(
CH
3
)
2
CH
}
2
C
=
NOC
{
(
CH
3
)
2
CH
}
2
N
=
O
+
H
2
O
+
2Ag
(6.6)
Similar kinetic behavior was found for the decay of
tert
-butylmethyliminoxyl.
8
The foregoing kinetic results encouraged the synthesis of di-
tert
-butyl ketoxime. This was no trivial
task because the only reported synthesis
17
required extremely high pressures, 125 000 psi (8300 atm) at
room temperature, which increased to 136 000 psi (9000 atm) at the reaction temperature of 75
◦
C. Fortu-
nately, the equipment required for such high pressure chemistry was available at the National Research
Council, though the amount of oxime that could be synthesized was very limited owing to the small size
of very high pressure reaction vessels. Di-
tert
-butyliminoxyl, generated from the ketoxime (Equations 6.1
and 6.2) in benzene in an EPR spectrometer at concentrations from 1
10
−
2
M, underwent
no measurable decay over the course of several days.
8
We therefore set out to synthesize a much larger
quantity of the ketoxime in order to prepare sufficient di-
tert
-butyliminoxyl to determine whether or not it
could be isolated, purified, and “put in a bottle”. This was made practicable by Hartzler's timely demon-
stration that hindered nitriles readily undergo nucleophilic attack.
18
10
−
6
-1
×
×
The following synthetic procedure
was developed.
19
6.2.1 Synthesis of di-
tert
-butyl ketoxime
Into 40 ml of 1.25 M
tert
-butyl lithium in
n
-pentane under nitrogen was added, drop-wise and with magnetic
stirring, 4.1 g (50 mmol) of pivalonitrile. After standing for one hour at 25
◦
C, there was added, successively:
5 ml absolute ethanol, 3 ml acetic acid, and 3.5 g hydroxylamine hydrochloride. An additional 65 ml of
absolute ethanol was used to transfer the mixture to a flask with an attached condenser, and the mixture
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